In general, a power steering system for an automotive vehicle comprises a servomotor which is controlled by the steering wheel for turning the dirigible wheels. Hydraulic servomotors are conventionally used and it is also known to utilize an electric servomotor.
In conventional vehicle steering systems, it is desirable for the dirigible wheels of the vehicle to automatically return to the straight ahead position after the driver has executed a turn and releases the steering wheel. In the conventional system, this is realized as the result of the geometry of the steering system. In particular, with the vehicle moving forward and the front wheels turned, the road forces acting on the front wheels tend to turn the front wheels back to the straight ahead position. This restoring force which originates from the reaction of the front wheels with the road surface, is transmitted through the steering linkage to the steering gear. Rotation of the steering gear to the straight ahead position is effective to drive the steering shaft and steering wheel to the straight ahead position. It also tends to drive the servomotor to the straight ahead position. In the case of an hydraulic servomotor the drag forces are compensated by changing the steering geometry to increase the restoring force. However, in the case of an electric servomotor, the resisting force due to friction in the servomotor is substantial in relation to the restoring force. Without special measures, the resisting force or drag imposed by the electric servomotor has the effect of slowing the return of the front wheels to the straight ahead position. The drag may be large enough that a manual assist through the steering wheel becomes necessary to turn the front wheels to the straight ahead position.
A general object of this invention is to provide a control system for reducing the drag of the servomotor and gear train on the return of the dirigible wheels to the straight ahead position to include this capability in a power steering system in a neat and compact structure and to overcome certain disadvantages of the prior art.